I thought I would offer some reassurance, while I'm currently going through a bad anxiety attack myself.
The one thing that never worries me anymore is the feeling of heart flutters, because after years I have actually discovered that they are diaphragm spasms.
They happen more when my stomach is very gassy, and when my heart goes fast. I also get them just in general when I'm really tense, or at night when I lie down particularly on my left side.

They feel like little bubbles in my chest. Very often they feel like my heart skips a beat and then gives a hard pound (like a PVC). But I've measured my heart a few times during this and it never skips a beat. That's when I did some research and found they were diaphragm spasms, which give many of the same feelings as skipped beats.
I feel like there's really not much to be found about that online as a possible reason for chest flutters, and I've never seen it posted on this forum.
So if I can offer anyone some solace with this, I'm happy to!